1. Field of the Invention
The present invention relates to an electromagnetic drive device for linearly reciprocatilvely moving an operating member such as, for example, a spool of a spool valve.
2. Discussion of the Related Art
Heretofore, as electromagnetic drive device for reciprocatively moving a spool of a spool valve, there has been known one described in Japanese unexamined, published patent application No. 1-242884 (1989-242884). In the known electromagnetic drive device, a first solenoid housing (i.e., core) and a second solenoid housing (i.e., yoke) both made of a magnetic material are arranged serially in axial alignment with a non-magnetic portion (i.e., air gap or non-magnetic member) placed therebetween, thereby to constitute a stator, and a plunger is slidably guided in an inner bore formed in the stator. By exciting the solenoid housings with a solenoid, the plunger is axially moved against a spring, so that a spool in a spool or valve housing attached to the first solenoid housing (i.e., core) is operated. Where the plunger is slidably received in the inner bore of the stator in this manner, a strict alignment is required between the internal surfaces of the yoke and the core. Therefore, it is necessary to machine the internal surfaces of the yoke and the core after they are inserted into and secured to a sleeve made of a non-magnetic material.
Further, there has also been known a technology described in U.S. Pat. No. 6,601,822 B2 to S. Tachibana et al. In this known technology, a stator for slidably guiding a plunger is constituted as a cylindrical stationary core which is made as one piece of a magnetic material, and a thin annular portion is formed by partly cutting out the outer wall portion at the axial mid position of the stationary core radially facing the plunger to the extent that the mechanical strength thereat is not deteriorated. A plurality of radial through holes are formed in the thin annular portion to decrease the area for magnetic path and thereby to increase the magnetic resistance thereat so that a portion equivalent to a non-magnetic portion can be formed at the thin annular portion.
Further, there is known a technology described in a technical journal “Materia Japan”, vol. 36, No. 4 (1997), pages 358-360. In this technology, a non-magnetic pipe made of a quasi-austenite base stainless steel is first converted by a cold roll process into a magnetic pipe, which is then partly processed by a selective quenching, whereby a magnetic stator with a non-magnetic portion at its axial mid portion can be made.
However, in the technology described in the aforementioned Japanese application, problems are raised in that the number of parts constituting the electromagnetic drive device increases and that many steps are needed for the machining of the fitting portions, press-fittings, and the finish machining of the inner bore for the plunger after the press-fittings, thereby resulting in an increase of the manufacturing cost. On the other hand, the problem of an increase in the manufacturing cost can be solved in the technology described in the aforementioned United States patent. That is, in the second technology, the annular portion is made thin and is provided with the plural radial through holes thereby to increase the magnetic resistance thereat. However, since it is unavoidable that the magnetic flux leaks through the annular portion, there is raised another problem that the magnetic attraction force exerted on the plunger is weakened. Further, the last mentioned technology for partly processing the magnetically converted stainless steel pipe by a selective quenching process needs plural steps of special processing, which undesirably results in an increase in the manufacturing cost.